The present invention relates generally to the field of micro-electromechanical switches, and, more particularly, to an apparatus and method for increasing the restoring force of a membrane particularly in the up direction.
Developments in micro-electromechanical systems (MEMS) have facilitated exciting advancements in the field of sensors (accelerometers and pressure sensors), micro-machines (microsized pumps and motors) and control components (high-definition TV displays and spatial light modulators). In addition, the micro-mechanical switches have advantages prominent semiconductor and over switch technologies for the routing of microwave and millimeter-wave signals. The routing of microwaves and millimeter wave signals is typically accomplished with gallium arsenide field-effect-transistors (FET) or p-i-n diode switches. These solid state devices can integrate comfortably with other high frequency electronics with low power loss. A disadvantage is the slow switching speed. However, there are a number of applications that do not need the high switching speeds and are more sensitive either to the losses in the switches or the power consumed by the switches. For these applications, micromechanical switches may be an attractive alternative to solid state switches. Electrostatically activated micromechanical switches can provide switching with low insertion loss, high isolation, very low power consumption, and unmatched linearity.
Recent developments in MEMS technology have made possible the design and fabrication of control devices suitable for switching microwave signals. Electrostatically actuated cantilever switches have been used to switch low-frequency electrical signals. Since these switches have demonstrated useful performance at microwave frequencies using cantilever, rotary and membrane topologies, these switches have shown that moving metal contacts possess low parasitics at microwave frequencies due to their small size and are amendable to achieving low on-resistance (resistive switching) or high on-capacitance (capacitive switching). This results in switches with very low loss, electrostatic actuation (no DC current required) and a potential for ultra-linear small-signal operation.
Micromechanical switches may have an active element in a thin metallic membrane movable through the application of a DC electrostatic field. A cross-sectional view of a membrane switch element in the unactuated state is illustrated in FIG. 1. The upper contact of the switch includes a 0.3-xcexcm or similarly sized aluminum membrane, suspended across polymer posts. Surface micromachining undercuts the post material from beneath the membrane, releasing it to be actuate. The suspended membrane typically resides 1-xcexcm or similarly sized above the substrate surface. On the substrate surface, a bottom contact includes a 0.7-xcexcm or similarly sized gold or aluminum first metal layer. On top of this first metal layer is positioned a thin dielectric layer, typically 1,000 xc3x85 or similarly sized layer of silicon nitride.
In the unactuated state, the membrane switch exhibits a high impedance due to the air gap between the bottom and top metal plates. Application of a DC potential between the upper and lower metal plates causes the thin upper membrane to deflect downwards due to the electrostatic attraction between the plates. When the applied potential exceeds the pull-in voltage of the switch, the membrane deflects into an actuated position. In this state, the top membrane rests directly on the dielectric layer and is capacitively coupled to the bottom plate. This capacitive coupling causes the switch to exhibit a low impedance between the two switch contacts. The ratio of the off- to on-impedances of the switch is determined by the on- and off-capacitances of the switch in the two switching states.
However, one of the problems with the device illustrated in FIG. 1 is that as this device cycles on and off, particularly at higher and higher frequencies, the device can get stuck with the membrane connected to the dielectric in the actuated state, leaving the device perpetually on. This is caused by the restoring force ({overscore (K)}MEMBRANE) not being sufficiently strong enough to release the membrane from the surface of the dielectric due to striction forces that work against the restoring forces.
The present invention provides a MEMS switch that minimizes the problems associated with sticking of the membrane with the dielectric, and, more particularly, the present invention provides pillars or supports that effectively reduce the radius of the membrane when the membrane has been collapsed as in the actuated state while in the dielectric layer is capacitively coupled to the bottom plate. In contrast, when the switch is off and the membrane is separated from the bottom plate, the radius is not reduced but enlarged since there is no effect of these pillars in the unactivated state. This maintains the requirement for a low pull down.